Inkjet printing mechanisms use cartridges, often called "pens," which shoot drops of liquid colorant, referred to generally herein as "ink," onto a page. Each pen has a printhead formed with very small nozzles through which the ink drops are fired. To print an image, the printhead is propelled back and forth across the page, shooting drops of ink in a desired pattern as it moves. The particular ink ejection mechanism within the printhead may take on a variety of different forms known to those skilled in the art, such as those using piezo-electric or thermal printhead technology. For instance, two earlier thermal ink ejection mechanisms are shown in U.S. Pat. Nos. 5,278,584 and 4,683,481, both assigned to the present assignee, Hewlett-Packard Company. In a thermal system, a barrier layer containing ink channels and vaporization chambers is located between a nozzle orifice plate and a substrate layer. This substrate layer typically contains linear arrays of heater elements, such as resistors, which are energized to heat ink within the vaporization chambers. Upon heating, an ink droplet is ejected from a nozzle associated with the energized resistor. By selectively energizing the resistors as the printhead moves across the page, the ink is expelled in a pattern on the print media to form a desired image (e.g., picture, chart or text).
To clean and protect the printhead, typically a "service station" mechanism is mounted within the printer chassis so the printhead can be moved over the station for maintenance. For storage, or during non-printing periods, the service stations usually include a capping system which hermetically seals the printhead nozzles from contaminants and drying. Some caps are also designed to facilitate priming, such as by being connected to a pumping unit that draws a vacuum on the printhead. During operation, clogs in the printhead are periodically cleared by firing a number of drops of ink through each of the nozzles in a process known as "spitting," with the waste ink being collected in a "spittoon" reservoir portion of the service station. After spitting, uncapping, or occasionally during printing, most service stations have an elastomeric wiper that wipes the printhead surface to remove ink residue, as well as any paper dust or other debris that has collected on the printhead.
To print an image, the printhead is scanned back and forth across a printzone above the sheet, with the pen shooting drops of ink as it moves. By selectively energizing the resistors as the printhead moves across the sheet, the ink is expelled in a pattern on the print media to form a desired image (e.g., picture, chart or text). The nozzles are typically arranged in one or more linear arrays. If more than one, the two linear arrays are usually located side-by-side on the printhead, parallel to one another, and perpendicular to the scanning direction. Thus, the length of the nozzle arrays defines a print swath or band. That is, if all the nozzles of one array were continually fired as the printhead made one complete traverse through the printzone, a band or swath of ink would appear on the sheet. The width of this band is known as the "swath width" of the pen, the maximum pattern of ink which can be laid down in a single pass. Any variation in the media-to-printhead spacing along the length of the nozzle array may yield visually acceptable deviations in print quality. There are a variety of different problems that make it difficult to always achieve consistent and accurate media-to-printhead spacing.
As a preliminary matter, there is a term of art used by inventors skilled in this art that will speed the reading if used herein, and it is "pen-to-paper spacing," often abbreviated as "PPS" or "PPS spacing." In the English language of the inventor, "pen-to-paper spacing" or "PPS" is easier to pronounce than the more technically explicit term "media-to-printhead spacing," and for this reason "pen-to-paper spacing" or "PPS" are used herein. During prototype testing and development, inventors use vast amounts of media, so the most plentiful and economical media, plain paper is used. Indeed, the short-hand term "pen-to-paper spacing" is a logical selection of terminology, although it must be understood that as used herein, this term encompasses all different types of media, unless specified otherwise in describing a particular type of media. Thus, "pen-to-paper spacing" (PPS) defines the spacing between the inkjet cartridge printhead and the printing surface of the media, which may be any type of media, such as plain paper, specialty paper, card-stock, fabric, transparencies, foils, mylar, etc. Having dispensed with preliminary matters, the discussion of the problems encountered in this art in maintaining an accurate PPS now continues.
First, there is a tendency for some graphic and photographic type images to saturate the media with ink, causing an undesirable effect known in the art as "cockle." The term "cockle" refers to the tendency of media, such as paper, to uncontrollably bend or buckle as the wet ink saturates the fibers of the media and causes them to expand. This buckling or cockling causes the media to uncontrollably bend either downwardly away from the printhead, or upwardly toward the printhead, with either motion undesirably changing the PPS spacing and leading to poor print quality. Moreover, upward buckling may be extreme enough to cause the media to actually contact the printhead, which may clog a nozzle and/or smear ink on the media, damaging the image.
Second, there are variations in the thickness of the print media which also affect the PPS spacing. For example, envelopes, poster board and fabric are typically thicker than plain paper or a transparency. The thicker media decreases the spacing from the printhead to the printing surface, and as with cockle, in the worst case, this reduced spacing could lead to contact of the printhead with the media, possibly damaging either the printhead or the image. Furthermore, these various media thicknesses also offer challenges to an automatic feed system, which must pick the top sheet from a stack of media, and then accurately feed it into the print zone.
One earlier media handling system tried to accommodate thicker envelopes, using a width sensor that detected media narrower than about 12 cm (4.5 in). Upon detecting this narrow media, a mechanical arm opened an inlet port on the media handling system to a much wider gap than normal to prevent ink smear on the envelope. Unfortunately, the assumption envelope was being printed just because the media width was narrow completely ignored the printing of postcards by a user. Thus, when printing postcards the print quality was severely degraded by the greater PPS spacing. Moreover, there was no provision for the user to defeat this mechanical widening of the gap when postcards where printed.
The earlier media handling systems lacked any ability to adjust the PPS spacing, other than adjustments made during initial assembly at the factory. Manufacturing adjustments are required to accommodate the large number of parts whose various tolerances accumulate and lead to a large degree of variability around the nominal spacing value. One earlier method involved the rotation of a helical cam, and the tightening of an adjustment screw to fasten the cam in place. Unfortunately, errors may occur during manufacturing, for example, from human error in reading a dial indicator measuring device or other display. Furthermore, the act of tightening the adjustment screw caused various mechanical stresses on the component parts. Additionally, physical access to the adjustment cam and screw had to be provided for in the mechanical design of the printer. Furthermore, this manual adjustment may occur when the printing mechanism was only partially assembled, so the addition of other parts to the printer mechanism could warp the spacing adjustment. Any of these inaccuracies in the PPS spacing occurring during manufacture could result in degraded print quality for the entire life of the printer.
Beyond the PPS spacing issue, the earlier media handling systems have suffered a variety of other disadvantages. Many of these earlier systems required a multitude of separate parts, for picking sheets of media from a stack, feeding the media through the print zone, and then depositing the printed sheet in an output tray. For example, one earlier design required 15-17 separate parts, which contributed significantly to the overall complexity and cost of the printing mechanism, not only in the actual cost of the parts themselves, but also in labor time required for their assembly. Additionally, many of these earlier media handling systems used spring loaded parts, which at some point during printing would snap the parts back into place; a noisy operation indeed. Most customers in the home or office environment want quieter printers, so this noise from return springs and the associated noise of the parts colliding with one another in the earlier designs was undesirable.
Given the criticality of the pen-to-paper spacing, the desire for higher print quality, which typically implies a closer spacing, as well as the ability to handle different types of media (e.g., envelopes, plain paper, card stock, etc.) and different images (e.g., text vs. graphic vs. photographic), it would be desirable to adjust the PPS spacing automatically during use. Such an automatic adjustment would also aid manufacturing, particularly if it could be implemented in a media handling system having fewer and quieter components.